For this, it's like finding a partial volume. Altogether, there will be 10 inches of sand, because 4+6 is 10. Because it is in inches, we have to convert it to feet. 10 inches in feet is 10/12 feet, or about .833. This is then used for the volume formula instead of the 1 foot height. So, the equation is 6 times 4 times 10/12. This comes out to be 20. Hope this helps :)
